The Climate of Tall Buildings
With increasing technology (such as elevators, air conditioning, and electric lightings) and development, the world has come to a decade where everyone wants modernized and attractive buildings (High rise buildings and or towers) to catch the attention of viewers, entrants and passers. Also, with the desire of most governments to expand their cities, most of which have highly occupied land surfaces, it is only possible that such expansions be executed by tall buildings.
In some tropical cities, the climate above the urban buildings is milder and not subject to the same humidity and temperature as it is at street level where solar radiation is absorbed by the ground. These improved comfort conditions may provide opportunities for low energy solutions such as passive cooling and hence providing outdoor living. In some other cities like in Hong Kong where residents’ desires for better views and increased access to daylight and fresh air have been a principle driver for increases in tall buildings.The higher levels of buildings are sought after. In some cities with cold climates such as in Moscow the lower levels of tall buildings can provide better living conditions where the higher levels over-heat due to vertical pressure differences causing mechanically warmed air to rise through stairs and shafts.
Some of the external conditions that vary with height include the surrounding building densities,day lighting levels and reflections from roof tops.
Tall buildings consist of many external and internal conditions that vary incrementally with increased height above the ground
Because architectural and environmental control solutions rarely respond to building height and instead provide uniform facades from the ground to roof level.
This article will tell you more about the climate of tall buildings.
Tall buildings are primarily located in urban and suburban locations and so the influence of the city in rural micro climates is important to the study of the climates of tall buildings.
Tall Buildings in groups cause their own micro-climates which are affected by urban geometry, surface thermal properties and human made waste heat inputs.
Whilst winds are light the boundary layer is mostly influenced by the ground conditions and can lead to significant vertical differences in climate character such as air temperature.
Also, solar radiation is a function of meso-climate which however can vary locally through changes in altitude, surface albedo and pollution levels. The absorption and emission of solar radiation by the ground surface forms the greatest effect on the lower atmosphere in a rural environment. Exposure to solar radiation is influenced by building clustering, density and height leading to shade and reflection from adjacent buildings. The areas of a tall building above the urban canopy layer will typically receive greater solar radiation than below the urban canopy which may be shaded for portions of the year due to greater exposure and reflections from adjacent rooftops. Additional solar radiation is received by vertical surfaces in urban areas from low altitude sun in the morning and evening than would be the case in a natural environment. The ability of building mass to cool at nights through radiation to the sky is reduced by the sky view factor meaning that denser building clusters will experience greater heat retention at nights.
In natural environments the temperature of the air layer near the ground is determined by the surface conditions, the transport of heat upwards and the extent of air mixing regardless of the effects of the surrounding area. This temperature is graded vertically from warmer at the surface to cooler higher in the atmosphere. This vertical gradient is usually known as the temperature lapse rate or the rate of decrease in temperature with height. It is important to note that daily patterns, temperature ranges and peak temperature times vary with the distance above the ground. Peak temperatures are delayed further into the afternoon with increasing height and with seasonal influences where they can be delayed by between 1 and 2 hours in winter. It should be noted that the variation of temperature lapse rate and the shifting of the peak temperature at heights up to 17m are much higher on bright days than cloudy days. Temperature is also affected by wind velocities particularly from the ground to a height of 2m where a lack of mobility of air allows the effects of solar radiation absorption and emission to be maintained.
In natural environments, humidity is influenced by the ground in a similar way to temperature where evaporation from the ground surface is directed upwards as vapor leading to decreasing humidity (or vapor pressure) with height. Measurements show that the humidity gradient decreases with additional height above the ground during the day and is subject to an inversion at night time whereby the humidity increases with height. The levels of humidity at ground level are subject to much higher variations. It is important to note that water vapor pressure decreases with height and being at lowest levels at 2.00pm in the afternoon and highest levels at 8.00am in the morning.
On the other hand, Wind velocity and direction is a result of large-scale differences in air pressure with the flow of air being from areas of higher pressure to areas of lower pressure. Local winds are small-scale winds produced by locally generated pressure gradients. Wind speeds typically increase with additional height and over the course of the day. At lower levels there is a distinct diurnal pattern in mid latitudes where wind speeds peak at midday and reduce at night time. At heights above 100 m the reverse is the case with wind speed peaking during the night and being minimized in the middle of the day.
Also, we have air pressure is the force exerted by the weight of air above and it decreases with additional height above sea level due to there being less air above to exert a force downwards. The vertical reduction in external air pressure causes the air to expand forming the basis for the vertical reduction in air temperature. In urban environment air pressure is further influenced by the effects of wind around building clusters creating positive and negative air pressure zones related to prevailing wind directions and speeds. In the case of tall buildings these differences in external air pressure influence the:
- Vertical movement of wind externally in the form of updrafts and downdrafts.
- Indoor air pressure differences, driving heat upwards through building voids with airflow moving from high to low pressure,
- Levels of infiltration and ex-filtration through the building envelope due to pressure differences across the exterior wall.